Pilot ACE

The Pilot ACE was one of the first computers built in the United Kingdom, at the National Physical Laboratory (NPL) in the early 1950s.

It was a preliminary version of the full ACE, which had been designed by Alan Turing. After Turing left NPL (in part because he was disillusioned by the lack of progress on building the ACE), James H. Wilkinson took over the project, Harry Huskey helped with the design. The Pilot ACE ran its first program on May 10, 1950 and was demonstrated to the press in December 1950.

Although originally intended as a prototype, it became clear that the machine was a potentially very useful resource, especially given the lack of other computing devices at the time. After some upgrades to make operational use practical, it went into service in late 1951, and saw considerable operational service over the next several years. One reason the ACE was useful is that it was able to perform Floating point arithmetic necessary for scientific calculations. Wilkinson tells the story of how this came to be. The pilot ACE was built without hardware for multiplication or long division, in contrast to other computers at that time. The ACE started out using fixed-point multiplication and division implemented as software. It soon became apparent that fixed point arithmetic was a bad idea because the numbers quickly went out of range. It only took a short time to write new software so that the ACE could do floating-point arithmetic. After that, James H. Wilkinson became an expert and wrote a book on rounding errors in floating-point calculations, which eventually became a big seller.

It had approximately 800 vacuum tubes, and used mercury delay lines for its main memory. The original size of the latter was 128 32-bit words, but that was later expanded to 352 words; a 4096-word drum memory was added in 1954. Its basic clock rate, 1 megahertz, was the fastest of the early British computers. The time to execute instructions was highly dependent of where they were in memory (due to the use of delay line memory). An addition could take anywhere from 64 microseconds to 1024 microseconds.

The machine was so successful that a commercial version of it, named the DEUCE, was constructed and sold by the English Electric Company.

The Pilot ACE was shut down in May, 1955, and was given to the Science Museum, where it remains today.